Researchers at the University of California-Berkeley claim to have hit upon a counterintuitive means of boosting the efficiency of flatplate solar cells by making them emit light. "What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce," said principal researcher, UC Berkeley Professor of Electrical Engineering Eli Yablonovitch.

To briefly recap the mechanism behind the photovoltaic effect itself, photons from some external light source (the sun, preferably) entering a solar cell excite the electrons in the semiconductor into higher energy states. This frees them from confinement so that they can convey current. (The charge itself is created by using two materials. Free electrons find it easier to move in one direction between the materials, creating a negative charge in one and a positive charge in the other.)

Berkeley researchers Eli Yablonovitch and Owen Miller

Eli Yablonovitch

But in some semiconductors, when the excited electrons return to lower energy states they have a knack for emitting photons, which is a desirable property for a semiconductor in, say, an LED. But Yablonovitch argues this is also crucial to solar cells—an argument his team is the first to make, a recent press release claims. Unlike an LED, the electrons in a solar cell are absorbing photons from an exterior source as well as emitting their own.

But the emitted photons find it hard to escape the semiconductor's surface due to their narrow escape cone. These trapped photons are likely to be re-absorbed, causing yet another subsequent photon emission. Yablonovitch asserts that the more photons that can be made to escape, the greater the voltage achievable in the cell. How? "Fundamentally, it's because there's a thermodynamic link between absorption and emission," explained team member Owen Miller.

Yablonovitch goes into more detail about this thermodynamic link in a 2011 paper (which, it should be pointed out, has yet to be peer reviewed). Due to "basic thermodynamics," a material that absorbs light must also emit light, to an extent determined by the material's absorptivity. "External photon emission is part of a necessary and unavoidable equilibration process," it explains, "which does not represent loss at all."

Yablonovitch has likened this to maximizing power from a water wheel driven by water from a faucet. In this analogy, the water tank has an open top and is continually filled by rainfall. If the faucet is fully open, the tank will drain too quickly, losing pressure and failing to turn the wheel. With a closed (or almost closed faucet), the pressure in the tank may be high, but the flow of water is insufficient to turn the wheel. The optimal setup is to have the faucet part open so that the water in and out of the tank is balanced and the pressure is equalized. You may be losing water, but the energy output of the water wheel over time is maximized.

The external luminescence from the solar may also be thought of as a gauge of the voltage in the cell. "My preferred way of explaining this is to say that the external luminescence is like a contactless volt-meter in the cell," he told Ars. "To say that we want more external luminescence is like saying we want more voltage."

An Alta Devices solar cell

Joe Foster, Alta Devices

How does one go about expelling these unwanted photons? A highly reflective rear mirror can help to expel new photons, as can an "optically textured" front surface that facilitates photon escape, the paper claims. And Yablonovitch has done exactly that, with the help of some friends.

In June, 2011, Alta Devices, a company cofounded by Yablonovitch, announced it had achieved an efficiency of 28.2 percent in its gallium arsenide-based solar panels (the previous record of 26.4 percent having been achieved in 2010).

The boost of almost two percent may sound modest, but when closing in on the Shockley-Queisser limit, every tenth of a percent counts. The Shockley-Queisser limit is the theoretical maximum efficiency—33.7 percent—at which single p-n junction flatplate cells can operate. Receiving 1000 W/m2 of solar radiation at noon on a clear day, these ideal cells would be capable of producing 337 W/m2 of electrical power. Multilayer cells are capable of greater efficiencies.

The voltage increase offered by the emission of photons may be sufficient to explain the shortfall between real-world achievements and the Shockley-Queisser limit—a limit which the team's research indicates should be perfectly possible in gallium arsenide, which has internal fluorescence approaching 100 percent. For the Shockley-Queisser limit to be achieved, solar cells require a 100-percent external fluorescence to "balance" the light coming in. In other words, a perfect system would emit one photon for every photon absorbed. Needless to say, a voltage increase equates to a power increase, since power (in watts) is the product of the current (in amps) and potential difference (in volts).

To achieve cell efficiencies greater than 30 percent, the optical performance of solar panels "will need to be very carefully designed," the 2011 paper, Intense Internal and External Fluorescence as Solar Cells Approach the Shockley Queisser Efficiency Limit, concludes. Yablonovitch hopes to see 30-percent efficiency cracked in the next few years. The team is set to present its latest findings in May in its presentation, The Opto-Electronics which Broke the Efficiency Record in Solar Cells, at the Conference on Lasers and Electro Optics in San Jose, California.

Ars Science Video >

A celebration of Cassini

A celebration of Cassini

A celebration of Cassini

Nearly 20 years ago, the Cassini-Huygens mission was launched and the spacecraft has spent the last 13 years orbiting Saturn. Cassini burned up in Saturn's atmosphere, and left an amazing legacy.

James Holloway
James is a contributing science writer. He's a graduate of the Open University, with a B.Sc. in Technology and a Diploma in Design and Innovation. Twitter@jamesholloway

First, I'll admit to not knowing all that much about solar tech, or even getting what all is in this article, but your opening picture had me wondering. Pretty much every organism that uses sunlight for energy has a color--ie, not black. A green plant absorbs most of sunlight, but reflects green--could this help explain why?

If a photon isn't consumed by interaction with a photovoltaic cell, couldn't the escaping photon be directed at another cell? I'm assuming that something happens to the photon that prevents it from doing so, but if were just reflecting the photon back out, why not send it to another cell that in turn reflects it out to another and so on. If we can bounce the same photon around and around we could get that much more energy out of them.

It's all well and good that they've upped the efficiency again, but the biggest problem right now in solar uptake is simply the -cost- of the systems. I'd be more than happy to put a system in place, and I have the perfect spot for it (south-facing garage roof), but at $4000/kwh (I'm just assuming I'd do a 3kwh system) PLUS installation it just isn't cost effective.

If it would/could come down to $1000/kwh (plus install), I'd do it in a heartbeat, and a huge majority of other people probably would as well as the return on investment would be quite worthwhile. This would bring out a massive rollout of solar power, and the resulting reduction in co2 and fossil fuel usage would be dramatic. Until it hits an affordable price point though, solar will remain niche, and pushing the boundaries on materials (exotic high-cost) isn't the way to do it. Find a relatively effective (20% efficiency would be fine), INEXPENSIVE way to do it, and the masses will come running - bringing profits with them.

The maximum energy harvest is reached when the photon that is leaving the cell is at thermal equilibrium with the surroundings. Cells normally run hot, which means that they are actually leaving at too high an energy level, carrying away some of the power with them. In any event, these photons are relatively low energy, generally in the IR range (otherwise they'd glow red). In that case, that photon may be suitable for re-capture in another cell, although it would be low energy and unlikely to be economical - you're getting 15% or so of a low-energy IR photon, not 15% of that juicy high-energy blue one.

As I understand Alta's work, their cell attempts to minimize the losses, but it's not clear *how*. A previous article suggested that they efficiently re-capture emitted photons, so that they leave with less energy. The image you can see on the monitor appears to show this process, with the red photons. Look at the example on the far left of the monitor: an external photon comes in, green, hits an atom to photo excite it, and also releases another photon heading to the left. That new photon creates another excitation, and another photon, which escapes. If that first emission had left the cell, the energy in the second excitation would be lost.

None of this is new, by the way. It was fully accounted for in the 1980s in several extensions of the original S-Q work. The Wiki article discusses this in some depth.

First, I'll admit to not knowing all that much about solar tech, or even getting what all is in this article, but your opening picture had me wondering. Pretty much every organism that uses sunlight for energy has a color--ie, not black. A green plant absorbs most of sunlight, but reflects green--could this help explain why?

The chloroplasts are optimized for red and blue but they absorb green too. They are slightly less efficient at it so about 5% less green light is absorbed then the red or blue, and that small percentage is enough to change the color.

hmm, do you think it's possible to engineer a street lamp to be used in this way? If we had a 1:1 ratio of light absorbed vs. light emitted, then there's no reason why all of our street lights couldn't really couldn't be used this way with of course built in LEDs for night time. (This also brings up another issue which I have yet to see a serious paper on, light pollution)

What's some of your guys opinions on how the excess light could be used? Greenhouses I could see be a great use for it too.

hmm, do you think it's possible to engineer a street lamp to be used in this way? If we had a 1:1 ratio of light absorbed vs. light emitted, then there's no reason why all of our street lights couldn't really couldn't be used this way with of course built in LEDs for night time.

Probably not. I doubt the photons being emitted are mostly in the visible spectrum; they would most likely to be of a lower energy than the photons being absorbed. So your cell could be soaking up sunlight all day and only give off infra-red photons. Useful for pit vipers, perhaps, but not so much for people (without IR-sensitive cameras).

It's all well and good that they've upped the efficiency again, but the biggest problem right now in solar uptake is simply the -cost- of the systems. I'd be more than happy to put a system in place, and I have the perfect spot for it (south-facing garage roof), but at $4000/kwh (I'm just assuming I'd do a 3kwh system) PLUS installation it just isn't cost effective.

If it would/could come down to $1000/kwh (plus install), I'd do it in a heartbeat, and a huge majority of other people probably would as well as the return on investment would be quite worthwhile. This would bring out a massive rollout of solar power, and the resulting reduction in co2 and fossil fuel usage would be dramatic. Until it hits an affordable price point though, solar will remain niche, and pushing the boundaries on materials (exotic high-cost) isn't the way to do it. Find a relatively effective (20% efficiency would be fine), INEXPENSIVE way to do it, and the masses will come running - bringing profits with them.

One of the things that is great about solar power is that there isn't just one market or one way to win. Yes, for home installation the installed cost per KWH is an important factor but there ends up being a lot of other costs besides just the panel cost for that. For the panels you can improve the cost per KWH by making the panels cheaper or by increasing the efficiency of the panel (assuming you maintain the other characteristics in both cases)

There are other markets like satellites where cost per KWH isn't a huge factor, you are already spending billions on the launch, but need higher efficiency and good power per weight.

So it’s pitch black all that time? No, it’s just not as bright as some other places.

They are SOLAR cells, not sunny cells. They work when it’s cloudy too, just less well.

If you've got cloud cover your solar cells are reduced in output by 40 to 90%, depending on how heavy the clouds are. Precipitation makes it even worse.

Solar pretty much is the worst form of green energy available to someone who lives in a place with routinely bad weather. You can still slap them on your roof and get a little benefit, but the cost/benefit equation gets much much worse and you absolutely need supplemental power from either the grid or another home generation source.

Still down around 12-13 cents here (advertised rate is lower but there are flat charges on top of that, that’s with 1 year contract). Ironically dropping below a certain threshold of monthly usage raises rate price somewhat, so would need to watch that doing a supplementary system. On the other hand if mounting the panels on the roof helped keep the attic cool that would bring it a lot closer as the bulk of Apr-Oct power usage is A/C related (SE Texas), with Dec, Jan, and Feb combined being about the same cost as July (or Aug) alone.

I expect the biggest problem though would be having to go up and clean off the mould and tree pollen.

EDIT: I’d count in the “For” column not having to worry about a back-up generator (minimum is about $500 plus having gasoline on hand for something that’ll keep the freezer and a single room window unit A/C running), as hurricane threat encourages that here. But I’m not sure how that works out with the risk of wind damage.

Ironically dropping below a certain threshold of monthly usage raises rate price somewhat, so would need to watch that doing a supplementary system.

Oh, now *there's* a good idea...

Yeah, I did a double-take when I read that the first time. I suspect it has to do with hidden flat costs and the provider (who is really a middleman utilizing power line owner’s infrastructure) trying to pare down prices as much as they can in price competition for the high volume customers.

I'm not being a grammar nazi here, I'm trying to understand the usage. "[A]n LED" was used twice in the article, and I always assumed that the usage would be the same as if the acronym was fully written out, in which case I thought it should be "a light emitting diode" and as such "a LED".

Sorry for the detour here, but I would like to correct my grammar if I am wrong.

Edit: and I should probably say here as well that I am greatly interested in solar articles like these. Thanks and keep them coming!

==tl;dr==The important thing is the nonradiative recombination probability, which can kind of be measured by observing the light extraction efficiency of an open-circuited solar cell. To actually generate usable power you do not want photons to escape your device

==Actual post==I could be missing something, but I'm not sure this paper says what it says it says. I've read through the arxiv proof -- it's pretty rough, but I think I have an understanding of their arguments.

First off, 2 and 3 DO improve solar cell efficiency, but NOT because they help get light out of the semiconductor. A reflective mirror is important because it keeps the photons bouncing around in the active area of the solar cell, giving us more chances to absorb a photon and do useful work. Textured surfaces are also used to minimize the reflection of photons from the front of the solar panel -- any photons that bounce off the surface of our solar panel don't actually make it inside, and can't be collected. It takes a while before a photon is actually absorbed, so our goal is to get as many photons into the device as possible, and keep them there as long as possible.

Claim 1 is interesting -- ultimately I think the important thing here is having a low probability of non-radiative recombination, which is what the authors are indirectly measuring with their open-circuit light extraction efficiency. Non-radiative recombination is unhelpful because we lose a free electron-hole pair, but we don't get a photon out so we don't get another shot at reusing that energy.

The open-circuit light-extraction efficiency is a nice way to measure the impact of non-radiative recombination, but enhancing light extraction is the last thing you want to do in a useful (not open-circuited) solar cell.

More detailThe authors support claim 1 by noting that a low extraction efficiency can be caused by a high number of non-radiative recombination events. This is true, as far as it goes. When a photon is absorbed in a semiconductor it produces an electron and a hole (think of it as a missing electron somewhere else). The electron and hole can recombine to re-emit a photon, in which case we're back to square one, or they can recombine in some other way (non-radiatively!), usually producing lots of heat and no usable energy. If we get a photon from our recombination event it bounces around in the semiconductor for a while, maybe being re-absorbed, maybe escaping the device entirely. If it takes a long time for the photon to escape the semiconductor, it may be absorbed and re-emitted many times, so if there's a significant chance of a non-radiative recombination, it's quite likely the photon will never make it back out.

I'm not being a grammar nazi here, I'm trying to understand the usage. "[A]n LED" was used twice in the article, and I always assumed that the usage would be the same as if the acronym was fully written out, in which case I thought it should be "a light emitting diode" and as such "a LED".

I’d have to track down my grammar bible (looks like it’s walked off) to be certain but I believe the rule is that if you pronounced LED as a word (sounds like “lead”) then ‘a’, otherwise use ‘an’ (remember that "el ee dee" is how it is pronounced).

I think it would be great if someone set up a loan system for solar cells where they take your power bill and use the difference due to solar to pay off the loan for the system. For example, say your average power bill was $200/month. You would agree to pay xyz $200/month. From that they would pay your powerbill and any left over would go to interest/principle. That way you don't see any hit to your pocketbook and helping the planet. When they are finally paid off, you take over paying your powerbill again, now at a reduced rate. Nothing up front, nothing extra during payoff.

Another thing. While they upped the efficiency "only 2%"--that is actually a 7% increase in performance.

I haven't read the article, but at what is stated here that for maximum efficiency for every photon you emit a photon of the same wavelength (as it seem implied) is clearly wrong.While it is true that adsorption and emission are connected, and high absorption means high emission, the emission depends on the temperature, and emission=adsorption only at equilibrium.In that case you cannot extract any energy (because you are at equilibrium), something that should be clear also from the naive thinking that the energy has to come from somewhere.On another topic the temperature on the sun is quite high...So in all real solar panels I expect emission < adsorption.

I'm not being a grammar nazi here, I'm trying to understand the usage. "[A]n LED" was used twice in the article, and I always assumed that the usage would be the same as if the acronym was fully written out, in which case I thought it should be "a light emitting diode" and as such "a LED".

Sorry for the detour here, but I would like to correct my grammar if I am wrong.

Edit: and I should probably say here as well that I am greatly interested in solar articles like these. Thanks and keep them coming!

"A"/"An" is a pronunciation aid, not a standalone grammar rule. If the following word is pronounced starting with a vowel sound, you use "An"; if not you use "A".

This is why you will see regional variants of "a humorous phrase" versus "an humorous phrase" (or, likely, "an humourous phrase" given the penchant for extra 'u's amongst those who leave 'h' silent).

So, there's no real rule there, and both "a LED" and "an LED" are correct; the writer is communicating their pronunciation of the acronym ("led" versus "L.E.D."), and there are no hard and fast rules of pronunciation of non-dotted acronyms so either way is correct.

While it is correct to say the paper urges (low-energy) photon escape for higher efficiency, "emit" is not the right term for this escape. The photons here are not being produced actively in the solar cell, but are a natural by-product of the photovoltaic process. They are "emitted" from the substrate no matter what; if they are allowed to escape, the paper contends that the efficiency of the cell is increased, whereas if they are not allowed to escape (ie, favoring heat generation within the cell) the efficiency of the cell falls.

Saying a solar cell must emit light gives the impression that they must have lightbulbs (or some other active light source) in them to work most efficiently, which is the opposite of the paper's assertion. A solar cell must allow low-energy photons to escape rather than produce heat or disrupt higher-energy photons reacting with the substrate. The main thrust (at least, unless I'm completely misreading it) is that low-energy photons need to be gotten rid of so they don't interfere with the efficient harvesting of high-energy photons' energy.

hmm, do you think it's possible to engineer a street lamp to be used in this way? If we had a 1:1 ratio of light absorbed vs. light emitted, then there's no reason why all of our street lights couldn't really couldn't be used this way with of course built in LEDs for night time.

Probably not. I doubt the photons being emitted are mostly in the visible spectrum; they would most likely to be of a lower energy than the photons being absorbed. So your cell could be soaking up sunlight all day and only give off infra-red photons. Useful for pit vipers, perhaps, but not so much for people (without IR-sensitive cameras).

... and I believe it only emits photons when it's absorbing photons which would make it useless for lamps anyway (unless for some weird reason, you need infra-red lights other than the sun during the day).

Even if we could power our homes and cars with solar power we would still need a ton of oil you can make so many products from crude that it would be hundreds of years until alternatives were found for all of them.

There are about 8 replies I'd like to make on this thread but only have limited time..

Echohead2 wrote:

A couple of things:

I think it would be great if someone set up a loan system for solar cells where they take your power bill and use the difference due to solar to pay off the loan for the system. For example, say your average power bill was $200/month. You would agree to pay xyz $200/month. From that they would pay your powerbill and any left over would go to interest/principle. That way you don't see any hit to your pocketbook and helping the planet. When they are finally paid off, you take over paying your powerbill again, now at a reduced rate. Nothing up front, nothing extra during payoff.

They have this, its called PACE or property-tax assisted clean energy. What happens is your local government (city/county) takes the loan out for you, and then you get solar power installed. From there, you pay it back in your property taxes each year for a certain period of time. The payback also transfers to any subsequent owners of the house - if you sell and move, the new owner has to continue to pay for PACE.

The problem with PACE is that, ironically, the GSEs (Fannie Mae, Freddie Mac) are the ones who are stopping PACE from being implemented. So the federal government, through the GSEs are working against one of the best tools we have to expand the reach of solar.

FWIW, in my area (southern Nevada), residential solar power installation is about $5.25/W. So a standard 3-5kW system would cost between $15,000-$26,000. But, the feds give you a 30% tax credit, so you'll get 30% of the price off your taxes (assuming you have that much tax burden). So the real costs are $12,000-$18,000. At 12c/kWh with the air conditioners running all summer long, my personal payback period is about 10-12 years. My only thing keeping me from installing it now is that I'm looking to move in the next few years.

Also, people need to stop equating solar with oil. They aren't interchangeable - even if you have an EV (and I do), you're going to charge it up at night, when you wont be getting solar power. The transmission grid generally isn't an issue with EVs in most suburban areas (the grid in most warm areas is already built to handle air conditioner loads, so at night when they aren't running, the EVs just take their place, but at an even lower level of consumption).

Edit: finally, I had a dream a few nights ago about someone figuring out how to get around the 33.7% limit, boosting efficiency to 65%. I've never wanted a dream to be real so much.